1. Field of the Invention
The present invention relates to a battery unit used as a power supply for, e.g., an electric vehicle.
2. Description of the Related Art
An electric vehicle is equipped with a battery unit as a power supply for running. The battery unit comprises a battery module, a battery case for storing the battery module, etc.
The battery needs to be charged. There are two ways of charging; one of which is a quick charge for charging in a short period of time, and the other uses a domestic power source that takes a longer time for charging than the quick charge. The battery module generates heat when the battery is quick-charged. There has thus been proposed a cooling structure in which a fan is provided in a battery case to bring cooling air into the battery case in order to cool off the battery module.
On the other hand, one great challenge of the electric vehicle is how to increase mileage. Therefore, the largest possible battery unit (including a plurality of battery modules) tends to be mounted. Thus, as the large number of battery modules are arranged in a limited space, the plurality of battery modules may be arranged side by side in the flowing direction of cooling air.
In this arrangement of the battery modules, while the battery modules disposed in the front part of the flowing direction of the cooling air are exposed to relatively cold cooling air, the battery modules disposed in the rear part of the flowing direction are exposed to the cooling air which has cooled off the front battery modules and thus increased in temperature. As a result, the battery modules disposed in the rear part are not well cooled off, so that the battery modules arranged in the battery case are not efficiently cooled off.
Thus, a bypass is formed in the battery case so that the cooling air may bypass the battery modules disposed in the front part of the flowing direction and may be guided to the rear part in a low-temperature state. In a cooling structure of this kind, a cooling air inlet is formed in the center of the front lower part of the battery case. The bypass is formed in the upper part of the battery case.
Furthermore, the distance between the battery modules adjacent in the width direction is gradually increased outward in the width direction so that the cooling air may equally flow in the width direction as well. This allows the cooling air to equally flow in the width direction as well. A structure of this type is disclosed in Japanese Patent No. 3085327.
However the cooling air is not well guided to the ends of a direction traversing a direction oriented from the inlet to outlet of the cooling air in the battery case, and the battery modules disposed at the ends thus tend to be not well cooled off.
In order to address this problem, in Patent document 1, the distance between the battery modules adjacent in the width direction is adjusted to equalize the flow of the cooling air in the width direction, so that the cooling air also flows to the ends.
However, in the structure in Patent document 1, a great distance has to be set between the battery modules disposed on the outer sides in the width direction. Therefore, the battery case is increased in size to allow for the above-mentioned distance when a large number of battery modules are mounted therein. An increase in the size of the battery case is not preferable.